Error concealment method for wireless communications

ABSTRACT

The invention relates to a method of reconstructing pixel values of a video frame for concealing corrupted pixel values. The method comprising receiving, by a receiving unit, of a signal from a communication channel and delivering by the receiving unit of video packets comprising pixel values possibly corrupted with errors; associating confidence levels with pixel values comprised in the video packets as delivered by the receiving unit; and reconstructing pixel values usable for display from the received pixel values, wherein a reconstructed value for a given pixel is obtained from the received values of a set of pixels, including the given pixel, weighted by their associated confidence levels. 
     The invention allows for better reconstruction of corrupted pixel values and reduces the perceived distortion when displaying the video frame.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit under 35 U.S.C. §119(a)-(d) ofUnited Kingdom Patent Application GB1112017.7, filed on Jul. 13, 2011and entitled “Error concealment method for wireless communications”.

The above cited patent application is incorporated herein by referencein its entirety

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to the field of wireless communication ofinformation and more particularly to wireless communication ofuncompressed video data.

2. Description of the Background Art

US patent application 2009/0021646 discloses a system and a method ofcommunicating uncompressed video information that facilitates theapplication of error concealment schemes particularly when a receiverhas limited buffering memory. The method consists in partitioningneighboring (spatially correlated) pixels into a predetermined number ofdifferent partitions and placing pixels from the different partitionsinto different packets for transmission. Then, if pixel information in areceived packet is corrupted (lost or damaged), one or more otherpackets which contain pixels that are spatially correlated to thecorrupt pixel(s) can be used to recover the corrupt pixel information.

The disclosed system provides a best effort error concealment schemethat uses pixel information contained in error free received packets torecover missing information.

The present invention has been devised to improve the error concealmenteffect with no additional information overhead compared to known errorconcealment schemes, particularly as taught by the above cited priorart.

SUMMARY OF THE INVENTION

To this end, the present invention provides according to a first aspectan apparatus for reconstructing pixel values of a video framecomprising:

a receiving unit adapted to receive a signal from at least onecommunication channel and to deliver video packets comprising pixelvalues possibly corrupted with errors;

a confidence estimation unit adapted to associate confidence levels withpixel values comprised in the video packets as delivered by thereceiving unit; and

a processing unit configured to reconstruct pixel values usable fordisplay from the received pixel values, wherein a reconstructed valuefor a given pixel is obtained from the received values of a set ofpixels, including the given pixel, weighted by their associatedconfidence levels.

According to this first aspect, pixel value which is detected ascorrupted or of insufficient quality is not discarded but is included inthe reconstruction. This advantageously makes it possible to use at bestall received information and leads to a better reconstructed pixel valueand reduces visual distortion at display.

Particularly, the processing unit is configured to select one of aplurality of possible reconstruction strategies in dependence upon theconfidence level associated with at least one pixel value of the set ofpixels.

In one implementation, the reconstructed value for the given pixel isequal to the received value of that given pixel if the confidence levelassociated with the received value of the given pixel is above a firstthreshold.

In another implementation, the reconstructed value for the given pixelis obtained by maximum ratio combining (MRC) received values of all theset of pixels if the confidence level associated with the value of thegiven pixel is below than or equal to the first threshold and above asecond threshold.

In another implementation, the reconstructed value for the given pixelis obtained by averaging the received values of all the set of pixels,excluding the given pixel, if the confidence level associated with thevalue of the given pixel is below than or equal to the second thresholdand the absolute value of the differences between the confidence levelsof pairs of pixels of the set, excluding the given pixel, are below thanor equal to a third threshold.

In another implementation, the reconstructed value for the given pixelis obtained by duplicating the received value of one pixel of the set,different from the given pixel and which value is associated with thehighest confidence level in the set, if the confidence level associatedwith the value of the given pixel is below than or equal to the secondthreshold and the absolute value of at least one difference between theconfidence levels of a pair of pixels of the set, excluding the givenpixel, is above the third threshold.

In another implementation, the reconstructed value for the given pixelis obtained by maximum ratio combining (MRC) received values of all theset of pixels if a cyclic redundancy check (CRC) indicates one or moreerrors in a video packet comprising the value of the given pixel and theconfidence level associated with that value of the given pixel is abovea second threshold.

In one implementation, the confidence levels associated with receivedpixel values are based on a signal-to-noise ratio (SNR) determined forsignal portions corresponding to the radio packets transporting,respectively, those pixel values.

In one implementation, the set of pixels is formed by spatiallycorrelated pixels in the video frame.

In one implementation, the set of pixels are transported over distinctcommunication channels and wherein distinct communication channelscorrespond to different transmission time slots and/or differenttransmission paths.

In one implementation, the pixel values are comprised in the videopackets in a raw video format.

In another implementation, the pixel values comprised in the videopackets are encoded in a compressed video format.

According to a second aspect, the present invention provides a method ofreconstructing pixel values of a video frame comprising:

receiving, by a receiving unit, of a signal from at least onecommunication channel and delivering by the receiving unit of videopackets comprising pixel values possibly corrupted with errors;

associating confidence levels with pixel values comprised in the videopackets as delivered by the receiving unit; and

reconstructing pixel values usable for display from the received pixelvalues, wherein a reconstructed value for a given pixel is obtained fromthe received values of a set of pixels, including the given pixel,weighted by their associated confidence levels.

According to a third aspect, the present invention provides a computerprogram product comprising a sequence of instructions which, whenexecuted on a processor, causes the processor to reconstruct pixelvalues usable for display of a video frame from received pixel values,wherein the received pixel values being comprised in video packetsdelivered by a receiving unit following a reception of a signal from atleast one communication channel and wherein a reconstructed value for agiven pixel is obtained from the received values of a set of pixels,including the given pixel, weighted by confidence levels associated withthe values of the set of pixels.

According to a fourth aspect, the present invention provides a computerreadable storage medium storing a program executable by a processor forreconstructing pixel values of a video frame, the program when executedcausing the processor to reconstruct pixel values usable for display ofa video frame from received pixel values, wherein the received pixelvalues being comprised in video packets delivered by a receiving unitfollowing a reception of a signal from at least one communicationchannel and wherein a reconstructed value for a given pixel is obtainedfrom the received values of a set of pixels, including the given pixel,weighted by confidence levels associated with the values of the set ofpixels.

According to a fifth aspect, the present invention provides a processingunit comprising:

a processor; and

a computer readable storage medium storing a program executable by theprocessor for reconstructing pixel values of a video frame, the programwhen executed causing the processor to reconstruct pixel values usablefor display of a video frame from received pixel values, wherein thereceived pixel values being comprised in video packets delivered by areceiving unit following a reception of a signal from at least onecommunication channel and wherein a reconstructed value for a givenpixel is obtained from the received values of a set of pixels, includingthe given pixel, weighted by confidence levels associated with thevalues of the set of pixels.

Another aspect of the present invention can provide a program which,when executed by a computer or processor in a receiving device, causesthe receiving device to carry out the method embodying the second aspectof the invention.

Yet further another aspect of the present invention can provide aprogram which, when executed by a computer or processor in a receivingdevice, causes the receiving device to function as the apparatusdescribed above according to the first aspect of the invention.

The program may be provided by itself, or carried by a carrier medium.The carrier medium may be a storage or recording medium, or it may be atransmission medium such as a signal. A program embodying the presentinvention may be transitory or non-transitory.

The particular features and advantages of the method, the computerprogram product, the computer readable storage medium and the processingunit being similar to those of the apparatus reconstructing pixel valuesof a video frame, they are not repeated here.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts for illustrative purposes a wireless communicationnetwork employing spatial diversity.

FIG. 2 illustrates a functional block diagram of a communication devicethat implements both a transmitter and a receiver.

FIG. 3 shows an example of an uncompressed video frame.

FIG. 4 depicts a time division multiplexing (TDM) used for sharingaccess to the radio medium.

FIGS. 5a, 5b, 5c and 5d illustrate different reconstruction strategiesused to conceal one line of corrupted pixels according to one embodimentof the invention.

FIG. 6 is a flowchart of a segment of a program that shows how pixelvalues of the video bitstream are reconstructed.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 depicts for illustrative purposes a wireless communicationnetwork 102 employing spatial diversity. Spatial diversity relies on theuse of a plurality of transmission paths between communicating devices.Typically, wireless communication network 102 operates in the unlicensed60 GHz frequency band (millimeter waves) for providing enough bandwidthcapacity to support the transport of uncompressed HD video content.

Network 102 comprises a first device 110 embodying a transmitter (Tx)and a second device 120 embodying a receiver (Rx). In this particularexample, the second device 120 is composed of a communication device 120a connected to the wireless network and of a display device 120 bconnected to the communication device 120 a for rendering the receiveddata content, e.g. displaying the video. It is to be noted that thefirst device 110 may also represent a relay device, the originator ofthe data being then represented by another device 100 belonging to thecommunication network. It is common to have a meshed network comprisingrelay devices for relaying data between different devices to cope withthe short range of the millimeter waves.

A signal emitted by antenna 111 of first device 110 may reach antenna121 of second device 120 through a line-of-sight (LOS) transmission pathP0 if it is not blocked by any obstacle. In addition, the signal may bereflected by objects 115 which may cause the establishment of aplurality of non line-of-sight (NLOS) transmission paths P1, P2 and P3.

Transmission paths P0, P1, P2, P3 may be created by different radiationpatterns/configurations of antenna 111 of first device 110 and detectedby different receiving patterns/configurations of antenna 121 of seconddevice 120. A narrow beam antenna (directional antenna) can be used atthe first device 110 when emitting a signal and/or at the second device120 when receiving a signal. Steering an antenna to a given orientationcorresponds to configuring its parameters (for example the weightingcoefficients associated with the elements of an antenna array) such thatthe radiation of the signal, in case of emission, or the antennasensitivity, in case of reception, is accentuated in that givendirection relatively to other directions.

In a variant implementation of the invention, each of the first device110 and second device 120 embodies both a transmitter and a receiver toestablish a bi-directional communication. This makes it possible forexample to insert feedback control information in the data flowtransmitted in the reverse direction from the second device to the firstdevice. In this implementation variant, the two devices share the samehardware platform. An apparatus based on this hardware platform isreferred to generically hereinafter as a communication device.

FIG. 2 illustrates a functional block diagram of a communication device200 that implements both a transmitter Tx and a receiver Rx.Communication device 200 includes a wireless transceiver(transmitter-receiver) 230, a link control unit 220 and an applicationunit 210, each of which is also coupled to a controller 240. Thecommunication device furthermore includes a ROM 250 and a RAM 260(computer readable storage medium) for data and program storage.

Typically, controller 240 is embodied as a central processing unit(CPU), which operates in accordance with a program stored in the ROM250. The controller provides a work area in the RAM 260, and accessesand uses the work area during operation.

The wireless transceiver 230 is typically radio frequency (RF)transceiver circuitry that is connected to an antenna 231. The RFtransceiver performs functions such as modulation/demodulation,signal-to-noise ratio (SNR) estimation and antenna control.

The link control unit 220 performs functions of media access control(MAC) and channel coding for protecting packets against channel errorsby encoding the packets using an error correction code at thetransmitter and decoding the received packets at the receiver.

When the communication device 200 is acting as a transmitter,application unit 210 generates video packets from a video bitstreamdelivered by a local or remote video source such as a HD player orset-top box for example. Optionally, source coding can be applied toindividually compress generated video packets. A preferred embodiment ofthe invention for packetizing the video stream into video packets isdescribed below with reference to FIG. 3.

When the communication device 200 is acting as a receiver, applicationunit 210 generates a video bitstream for display or storage for examplefrom received video packets comprising pixel values possibly corruptedwith errors. The way pixel values of the generated bitstream arereconstructed from those received in the video packets is describedbelow with reference to FIGS. 5 and 6 according one embodiment of theinvention. The reconstruction of the bitstream pixel values includes theconcealment of corrupted pixel values received in video packetsaccording to an embodiment of the present invention.

Controller 240 will normally control overall data processing over thereceived or to be transmitted video data, whereas signal processingoperations associated with communication functions are typicallyperformed in RF transceiver circuitry 230.

Referring to FIG. 3, a method of packetizing video data according to apreferred embodiment of the invention is now described.

FIG. 3 shows an example of an uncompressed video frame 300 formedtypically of 1080 lines (vertical axis 302) of 1920 pixels each(horizontal axis 301). Video frame pixels are grouped into blocks 310 ofspatially correlated (neighbouring) pixels. Preferably, pixels of eachblock, referred to as adjacent pixels, are partitioned to differentvideo packets and each video packet is transmitted in a differentcommunication channel.

In the illustrated example, each of the pixel blocks 310 includes threeadjacent pixels 311, 312 and 313 which are represented by differentsymbols. The three partitions of the different blocks are included inthree different packets 331, 332 and 333. The positions 321, 322 and 323represent the values of pixels 311, 312 and 313 of one pixel blockrespectively. Video frame 300 is divided into 15 groups 340, 350, . . ., 360 of 72 lines each. Pixel values of first group 340 are alltransported in the three packets 331, 332 and 333. Pixels values ofsecond group 350 are similarly partitioned to a second set of threepackets (not represented) and these three packets are transmitted overdifferent communication channels, and so on. In the present example,pixel values of the whole frame are transported into 15×3 data packets.Other partitioning of the video frame can of course also be envisagedsuch as grouping video frame pixels into square blocks of 4 or 9 pixels.

The partitioning of pixels into different data packets allows for a moreefficient reconstructing of one degraded pixel value from the values ofits adjacent pixels as it is unlikely that all packets are to beaffected similarly by transmission errors. The process of reconstructingpixel values will be described in more detailed hereinafter.

The transmission of the video packets over the physical medium iscontrolled by the link control unit 220. Necessary protocol overheadinformation is appended to the video packets, in header and possiblytrailer parts, thus forming physical packets, also referred to as radiopackets when transmitted over a radio communication channel (cf.reference 451 in FIG. 4).

The setting up of a plurality of transmission paths in the wirelesscommunication network 102 is advantageously used in the presentinvention to create a plurality of communication channels between thetransmitter Tx and the receiver Rx over which are transmitted the radiopackets.

In one embodiment of the invention, time division multiplexing (TDM) isused for sharing access to the radio medium as depicted in FIG. 4. Aplurality of time slots 421, 422, 423, . . . are provided periodicallyin every frame 410, 420, 430. The start of a frame is signaled by meansof a beacon signal 440 consisting of a predetermined pattern of datasymbols. A communication channel is created by associating one giventime slot of a series of frames, e.g. 421, with one given transmissionpath. Sending radio packets, e.g. 451, over said communication channelcorresponds to configuring transmitter antenna 111 to radiate in atleast the direction of the given transmission path and emitting radiosignals representative of said radio packets during the given time slot421 in the series of frames. Receiving data from said communicationchannel corresponds to configuring receiver antenna 121 to be sensitivein at least the direction of the given transmission path and receivingradio signals representative of said data during the same given timeslot in the series of frames. Consequently, different communicationchannels correspond to different transmission time slots and/ordifferent transmission paths.

In another embodiment of the invention, a frequency division multipleaccess (FDMA) scheme may be used for sharing the radio medium. Acommunication channel is then created by associating one given carrierfrequency with one given transmission path. Sending data over saidcommunication channel corresponds to configuring transmitter antenna 111to radiate in at least the direction of the given transmission path andemitting radio signals representative of said radio packets bymodulating the given carrier frequency. Receiving radio packets fromsaid communication channel corresponds to configuring receiver antenna121 to be sensitive in at least the direction of the given transmissionpath and receiving signals representative of said radio packets bydemodulating the given carrier frequency. Consequently, differentcommunication channels correspond to different carrier frequenciesand/or different transmission paths.

In a further embodiment, the two above embodiments are combined. Acommunication channel is created by associating one given transmissionpath with both one given time slot of a series of frames and one givencarrier frequency. Consequently, different communication channelscorrespond to different carrier frequencies and/or differenttransmission time slots and/or different transmission paths.

By way of a broad overview of the invention, a pixel whose value isdetected to be corrupted or of insufficient quality is reconstructedbased on the values of a set of n pixels, including the pixel to bereconstructed, weighted by estimated confidence levels associated witheach value of the set of pixels. In a preferred embodiment of theinvention, the set of pixels corresponds to a block of pixels, e.g.pixel blocks 310 in FIG. 3, where each pixel of the block is partitionedinto a different video packet.

Let's consider Vg as the pixel value to be reconstructed into a valueV′g to conceal the presence of errors introduced during the transmissionof pixel Pg, where g is an index comprised between 1 and n. We have:

${V_{g}^{\prime} = \frac{\sum\limits_{j = 1}^{n}{V_{j}Q_{j}}}{\sum\limits_{j = 1}^{n}Q_{j}}};$

where Qj represents a quality factor or more generally a confidencelevel associated with a pixel value Vj and n the number of pixels in theset. The higher the confidence level, the lower the number of errors andthe better is the quality.

In contrast to conventional methods, a pixel value which is detected ascorrupted or of insufficient quality is not discarded but is included inthe reconstruction. This advantageously allows to use at best allreceived information and leads to a better reconstructed pixel value.

Confidence levels associated with a pixel value can correspond to theinverse residual bit error rate (BER) of pixel data (luminance dataand/or chrominance data) after channel error decoding. This pixel BERcan be taken equal to the BER of the radio packet comprising that pixelvalue. The lower is the BER, the higher is the level of confidence.Calibrating confidence level thresholds based on BER can be performed onthe perceived distortion induced by corrupted pixels.

In an alternate embodiment, confidence levels associated with a pixelvalue can correspond to the signal-to-noise ratio (SNR). Similarly, theSNR associated with a given pixel value can be taken equal to the SNR ofthe received signal corresponding to the radio packet comprising thatgiven pixel value. The higher is the SNR, the higher is the level ofconfidence.

It should be noted that for a given channel coding and modulationscheme, it is well known in the art to establish a relation (curve)between the BER and the SNR. This allows for calibrating thresholds whenusing directly the SNR by referring to the induced residual bit errorrate and thus of the perceived distortion induced by corrupted pixels.

A SNR estimation means is usually implemented by the RF transceiver unit230. As an implementation example, the SNR estimation can be performedaccording to the following principle.

Most digital modulations map the data to a number of discrete points onthe I/Q (magnitude/phase) plane known as constellation points. Becauseof noise, the magnitude/phase of a sample of received signal wouldlikely not coincide with the theoretical constellation pointcorresponding to the transmitted signal sample. The distance between theactual and theoretical constellation points is indicative of the noisestrength. This can be expressed as follows:

${N = \frac{\sum\limits_{i = 1}^{k}{\min_{i}\left\lbrack \left( {S_{i}^{*} - S_{i}} \right)^{2} \right\rbrack}}{k}};$

where N is the noise power, k is the number of signal samplescorresponding to one radio packet, min is the minimum distance betweenthe constellation point of a received sample Si* and the closesttheoretical constellation point Si.

It should be noted that the comparison is done by applying the maximumlikelihood criteria, which means that the closest theoreticalconstellation point is used instead of the constellation pointcorresponding to the actually transmitted signal sample which is notavailable.

After estimating the noise power affecting the received signalcorresponding to a radio packet, the signal to noise ratio can beestimated as follows:

${S\; N\; R} = \frac{\sum\limits_{i = 1}^{k}\left( S_{i}^{*} \right)^{2}}{N \cdot k}$

FIGS. 5a, 5b, 5c and 5d illustrate different reconstruction strategiesused to conceal one line of corrupted pixels (512 a, 512 b, 512 c and512 d) according to one embodiment of the invention. Same principleapplies for the reconstruction of either one pixel value or the valuesof a line of pixels as it is assumed that pixels of one line belong to asame partition, transmitted in a same radio packet and being subject toa same SNR. As an example, the SNR is used here as a measure of theconfidence level.

In FIG. 5a , the SNR of the radio packet corresponding to the line 512 a(line m) is assumed to be sufficiently good, i.e. SNR(m)>Th1, where Th1corresponds to a SNR threshold above which no or a negligible number oferrors remain in the video packet after channel decoding of the radiopacket. In this situation no concealment is necessary and the line pixelvalues usable for display of line 512 a are kept unchanged from thereceived pixel values in the video packet.

In FIG. 5b , the SNR of the radio packet corresponding to the line 512 b(line m) is slightly degraded, i.e. Th1≧SNR(m)>Th2, where Th2corresponds to a SNR threshold above which useful information is stillpresent after channel decoding of the radio packet. In this situation, afirst reconstruction strategy is applied where reconstructed pixelvalues usable for display of line 512 b are determined by maximum ratiocombining the pixel values of the three partitions (lines m−1, m, m+1)as follows:

${V_{i,m}^{\prime} = {\frac{\sum\limits_{j = 1}^{n}{V_{i,j}Q_{i,j}}}{\sum\limits_{j = 1}^{n}Q_{i,j}} = \frac{{V_{i,{m - 1}}S\; N\; R_{m - 1}} + {V_{i,m}S\; N\; R_{m}} + {V_{i,{m + 1}}S\; N\; R_{m + 1}}}{{S\; N\; R_{m - 1}} + {S\; N\; R_{m}} + {S\; N\; R_{m + 1}}}}};$

where V′i,m represents the reconstructed value of pixel i of line m,Vi,j represents the value of pixel i of line j, Q_(i,j), represents theconfidence level of pixel i of line j and SNRx the SNR associated withline x (all pixels of line x are assumed to have the same SNR becausetransmitted in the same radio packet). The reconstructed value of eachgiven pixel i of line m is obtained from the received values of the setof three pixels i of lines m−1, m and m+1, thus including the givenpixel i of line m, weighted by their associated confidence levels (SNR).

In FIG. 5c , the SNR of the radio packet corresponding to the line 512 c(line m) is too degraded, i.e. Th2 SNR(m) and no useful information canbe expected to be present after channel decoding of the radio packet. Ifthe two adjacent lines m−1 and m+1 are of similar quality, i.e.|SNR(m−1)−SNR(m+1)|≦Th3, where Th3 can be set for example to 10% of Th1,a second reconstruction strategy is applied where reconstructed pixelvalues usable for display of line 512 c are determined by averaging thepixel values of two partitions corresponding to lines m−1 and m+1 asfollows:

${V_{i,m}^{\prime} = {\frac{\sum\limits_{{j = 1},{j \neq m}}^{n}V_{i,j}}{n - 1} = \frac{V_{i,{m - 1}} + V_{i,{m + 1}}}{2}}};$

In a variant implementation, the two partitions are weighted by theircorresponding SNR:

${V_{i,m}^{\prime} = {\frac{\sum\limits_{{j = 1},{j \neq m}}^{n}{V_{i,j}Q_{i,j}}}{\sum\limits_{{j = 1},{j \neq m}}^{n}Q_{i,j}} = \frac{{V_{i,{m - 1}}S\; N\; R_{m - 1}} + {V_{i,{m + 1}}S\; N\; R_{m + 1}}}{{S\; N\; R_{m - 1}} + {S\; N\; R_{m + 1}}}}};$

where V′i,m represents the reconstructed value of pixel i of line m,Vi,j represents the value of pixel i of line j, Q_(i,j), represents theconfidence level of pixel i of line j and SNRx the SNR associated withline x.

In FIG. 5d , the SNR of the radio packet corresponding to the line 512 d(line m) is too degraded, i.e. Th2 SNR(m) and no useful information canbe expected to be present after channel decoding of the radio packet,similarly to the case of FIG. 5c . In this scenario however it isassumed that the two adjacent lines m−1 and m+1 are of differentquality, i.e. |SNR(m−1)−SNR(m+1)|>Th3. In this situation, a thirdreconstruction strategy is applied where reconstructed pixel valuesusable for display of line 512 d are determined by selecting the pixelvalues, among the two partitions corresponding to lines m−1 and m+1,having the best SNR. In the illustrated example of FIG. 5d , linem+1(513 d) is assumed to correspond to the radio packet with the bestSNR, and its pixel values are thus duplicated into line m (512 d).

As previously described, the controller 240 operates in accordance witha program stored in the ROM 250. FIG. 6 is a flowchart of a segment ofthe program stored in the ROM 250 of second device 120 and shows howpixel values of the video bitstream are reconstructed according to thedifferent reconstruction strategies described with regards to FIG. 5.

At step S601, packets including pixel partitions of one or more pixelblocks are received from a receiving unit including the transceiver unit230 and optionally the link control unit 220 for applying channel errordecoding to the radio packets.

At step S602, the SNR of received packets is estimated for example bythe SNR estimation means of the transceiver unit 230 using to the abovedescribed method.

A test is performed at step S603 to check whether the SNR of thedifferent radio packets including all pixel partitions are all toodegraded, i.e. SNR<Th2, where SNR threshold Th2 being as defined above.If the test is positive, it is not possible to apply the reconstruction,at least based on the pixels belonging to the set or block of pixels. Itis possible to envisage here to rely on pixel values not belonging tothe block of pixels if received in packets with sufficient SNR.Alternatively, a failure is detected (S613) and no reconstruction isperformed.

If at least one packet has a SNR above than or equal to threshold Th2,at step S604 the SNR of that packet is assigned to the pixel valuesincluded therein.

A test is performed at step S605 to check whether the SNR of a givenpixel to be reconstructed is sufficiently good, i.e. SNR>Th1, thresholdTh1 being as defined above. In this situation, no concealment isnecessary and the value usable for display of the given pixel is keptunchanged from the received pixel value in the video packet (S615).

In an alternate implementation, radio packets are individually protectedby an error detection code, e.g. by adding a checksum or a cyclicredundancy check (CRC). In this implementation, test S605 is consideredas positive if the checksum of the received radio packet comprising thegiven pixel value is determined as valid and thus no error are detected.

If SNR≦Th1 or the checksum validation has failed, a test is performed atstep S606 to check whether the SNR of the given pixel to bereconstructed is slightly degraded, i.e. SNR>Th2, where SNR thresholdTh2 being as defined above. In this situation, the first reconstructionstrategy is applied where the reconstructed value usable for display ofthe given pixel is determined by maximum ratio combining the differentpixel values of the set or block as detailed above (S616).

If SNR≦Th2, a test is performed at step S607 to check whether the valuesof other adjacent block pixels are of similar quality, i.e. whether wehave |Diff(SNR adjacent pixels)|≦Th3, where SNR threshold Th3 is asdefined above. If the test is positive, the second reconstructionstrategy is applied where the reconstructed value usable for display ofthe given pixel is determined by averaging the values adjacent pixels ofthe block (S617). If the test is negative, the third reconstructionstrategy is applied where the reconstructed value usable for display ofthe given pixel is determined by identifying among the values ofadjacent pixels the one having the best SNR (S608) and selecting thatpixel value for reconstructing the given pixel (S618).

If the number of pixels per block is greater than three, at step S607 atest is performed to check whether the absolute value of all differencesbetween the confidence levels of pairs of pixels of the set, excludingthe given pixel, is below than or equal to threshold Th3. If the test ispositive, the second strategy is applied (S617). If the test isnegative, i.e. the absolute value of at least one difference between theconfidence levels of a pair of pixels of the set, excluding the givenpixel, is above threshold Th3, then the third strategy is applied(S618).

The invention claimed is:
 1. An apparatus for reconstructing pixelvalues of a video frame comprising: a receiving unit adapted to receivea signal from at least one communication channel, the signal comprisingvideo packets comprising pixel values possibly corrupted with errors; aconfidence estimation unit adapted to associate confidence levels withpixel values comprised in the video packets received by the receivingunit; and a processing unit configured to reconstruct pixel valuesusable for display from the received pixel values, wherein areconstructed value for a given pixel is obtained from the receivedvalues of a set of pixels, including the given pixel, weighted by theirassociated confidence levels, wherein the reconstructed value for thegiven pixel is obtained by: maximum ratio combining (MRC) receivedvalues of all the set of pixels if the confidence level associated withthe value of the given pixel is below than or equal to a first thresholdand above a second threshold, and averaging the received values of allthe set of pixels, excluding the given pixel, if the confidence levelassociated with the value of the given pixel is below than or equal tothe second threshold and the absolute value of the differences betweenthe confidence levels of pairs of pixels of the set, excluding the givenpixel, are below than or equal to a third threshold.
 2. An apparatusaccording to claim 1, wherein the reconstructed value for the givenpixel is equal to the received value of that given pixel if theconfidence level associated with the received value of the given pixelis above the first threshold.
 3. An apparatus according to claim 1,wherein the reconstructed value for the given pixel is obtained byduplicating the received value of one pixel of the set, different fromthe given pixel and which value is associated with the highestconfidence level in the set, if the confidence level associated with thevalue of the given pixel is below than or equal to the second thresholdand the absolute value of at least one difference between the confidencelevels of a pair of pixels of the set, excluding the given pixel, isabove the third threshold.
 4. An apparatus according to claim 1, whereinthe reconstructed value for the given pixel is obtained by maximum ratiocombining (MRC) received values of all the set of pixels if a cyclicredundancy check (CRC) indicates one or more errors in a video packetcomprising the value of the given pixel and the confidence levelassociated with that value of the given pixel is above a threshold. 5.An apparatus according to claim 1, wherein the confidence levelsassociated with received pixel values are based on signal-to-noiseratios (SNR) determined for signal portions corresponding to the radiopackets transporting, respectively, those pixel values.
 6. An apparatusaccording to claim 1, wherein the set of pixels is formed by spatiallycorrelated pixels in the video frame.
 7. An apparatus according to claim6, wherein values of the set of pixels are transported over distinctcommunication channels and wherein distinct communication channelscorrespond to different transmission time slots and/or differenttransmission paths.
 8. An apparatus according to claim 1, wherein thepixel values are comprised in the video packets in a raw video format.9. An apparatus according to claim 1, wherein the pixel values comprisedin the video packets are encoded in a compressed video format.
 10. Amethod of reconstructing pixel values of a video frame comprising:receiving a signal from at least one communication channel anddelivering video packets, the signal comprising pixel values possiblycorrupted with errors; associating confidence levels with pixel valuescomprised in the received video packets; and reconstructing pixel valuesusable for display from the received pixel values, wherein areconstructed value for a given pixel is obtained from the receivedvalues of a set of pixels, including the given pixel, weighted by theirassociated confidence levels, wherein the reconstructed value for thegiven pixel is obtained by: maximum ratio combining (MRC) receivedvalues of all the set of pixels if the confidence level is associatedwith the value of the given pixel is below or equal to a first thresholdand above a second threshold, and averaging the received values of allthe set of pixels, excluding the given pixel, if the confidence levelassociated with the value of the given pixel is below than or equal tothe second threshold and the absolute value of the differences betweenthe confidence levels of pairs of pixels of the set, excluding the givenpixel, are below or equal to a third threshold.
 11. A non-transitorycomputer readable storage medium storing a program comprising executableinstructions causing a computer to perform the method of claim
 10. 12. Amethod according to claim 10, wherein the reconstructed value for thegiven pixel is further obtained by duplicating the received value of onepixel of the set, different from the given pixel and which value isassociated with the highest confidence level in the set, if theconfidence level associated with the value of the given pixel is belowthan or equal to the second threshold and the absolute value of at leastone difference between the confidence levels of a pair of pixels of theset, excluding the given pixel, is above the third threshold.